Final Report for Grant No. DE-FG02-01ER63220 "The Dynamics of Cellular Stress Responses in Deinococcus radiodurans"

Bacteria belonging to the family Deinococcaceae are some of the most ionizing radiation (IR) resistant organisms yet discovered. Deinococcus radiodurans is obligate aerobic, capable of growth under chronic IR (60 Gy/hour) and relatively resistant to many DNA damaging conditions including exposure to desiccation, ultraviolet radiation and hydrogen peroxide. The genes and cellular pathways underlying the survival strategies of D. radiodurans have been under investigation for fifty years. In the last decade, D. radiodurans was subjected to whole-genome sequencing, annotation and comparative analysis, whole-transcriptome and whole-proteome analyses, and numerous DNA repair studies. Collectively, published reports support that the key to survival of D. radiodurans resides in its ability to repair DNA, but the mechanisms responsible remain poorly defined. Unexpectedly, many novel genes implicated in recovery from IR by transcriptome and proteome profiling have had little effect on survival when disrupted, and there is reason to ask if something is missing from classical models of radiation resistance. The prevailing dogma of radiation toxicity has been that the cytotoxic and mutagenic effects of radiation are principally the result of DNA damage that occurs during IR. However, in light of available whole genome sequences, one broad observation that is difficult to reconcile with this view is that many organisms that encode a compliment of DNA repair and protection functions are killed at radiation doses that cause little DNA damage. This indicates that there are cellular targets involved in recovery that are more vulnerable to IR damage than DNA. It has been reported that D. radiodurans and other resistant organisms accumulate very high intracellular concentrations of Mn(II), and restricting the amount of Mn(II) during recovery from IR substantially reduced survival of D. radiodurans. At high intracellular concentrations, Mn(II) is known to act as a true catalyst of the dismutase of superoxde (O2?-), with Mn cycling between the divalent and trivalent states. Superoxide is generated during water radiolysis, particularly in the presence of iron redox-cycling processes, and has been implicated in damaging [4Fe-4S] cluster-containing proteins, with the release of bound Fe(II). Thus, it is possible that Mn(II) accumulation acts as keystone among antioxidant defenses which limits protein damage during both irradiation and recovery, with the result that DNA repair and other enzymic systems involved in recovery function with greater efficiency in D. radiodurans than other organisms